Plasmid maintenance in E.coli will be studied by determining the in vivo transcription pattern of plasmid DNA. Two types of plasmids will be studied, R factors which confer multiple antibiotic resistance on cells which harbour them, and bacteriophage Pl which exists as a plasmid in lysogenic cells. Transcription of the plasmid DNA will be analyzed by DNA-RNA hybridzation. Restriction nucleases will be used to prepare a physical map of these plasmids, and the transcription of individual fragments will be determined by DNA-RNA hybridization. Mutants which effect the level of conjugal transfer or antibiotic resistance will also be studied to determine the role of transcriptional control in the maintenance and transfer of R plasmids. Mutants which effect plasmid maintenance will be isolated and characterized, biochemically and genetically, to determane the nature and extent of the chromosomal genes involved in plasmid maintenance.